The radiated output power of mobile terminals, such as mobile phones, Personal Digital Assistants (PDAs), and the like, is based on forward power provided to an antenna by a power amplifier. The forward power is a function of the total power delivered by the power amplifier minus any power reflected back from the antenna due to impedance mismatches from the power amplifier to the antenna. The relationship of forward power to reflected, or reverse, power may be characterized using a voltage standing wave ratio (VSWR). For an open loop power amplifier, the total delivered power is a function of the load impedance seen at the output of the power amplifier. Ideally, the load impedance is a constant load. However, in reality, the load impedance may vary due to variations in the impedance of the components in the transmit chain following the power amplifier, such as impedance matching circuitry or a duplexer, caused by frequency, temperature, and process variations. The load impedance may also vary due to variations in the impedance of the antenna of the mobile terminal caused by proximity of the antenna to foreign metallic objects and the user's body.
These impedance variations and resulting VSWR variations may reduce the delivered power from the power amplifier and may reflect a portion of the delivered power back to the power amplifier, thereby reducing the forward power fed into the antenna. Therefore, to maintain a relatively constant radiated power from the antenna, the forward power may be measured and then regulated by controlling the output power of the power amplifier. VSWR measurements may be made by measuring both the forward and reverse power.
Directional power detectors, also known as directional couplers, are used to measure forward or reverse power and are traditionally coupled between impedance matching circuitry and the antenna; however, a traditional directional coupler may introduce harmonics, which must be removed with harmonics filtering that is coupled between the directional coupler and the antenna. Coupling the directional coupler between the power amplifier and the impedance matching circuitry may eliminate the need for the harmonics filtering; however, a traditional directional coupler may present an unacceptably high impedence in series with the low output impedance of the power amplifier. Traditional directional couplers may have difficulty differentiating forward from reverse power when presented with a complex load impedance, such as that presented by impedance matching circuitry. Thus, there is a need for a low series impedance directional power detector that can accurately differentiate forward from reverse power when presented with a complex load impedance.